Miniature cameras are becoming increasingly common in mobile electronic devices such as smartphones. For such high-end miniature cameras, it is common to incorporate autofocus (AF), whereby the object focal distance is adjusted to allow objects at different distances to be in sharp focus at the image plane and to be captured by the digital image sensor. There have been many ways proposed for achieving such adjustment of focal position, however most common is to move the whole optical lens as a single rigid body in a direction parallel to the optical axis. Positions of the lens closer to the image sensor correspond to object focal distances further from the camera.
Demands on improvements to performance of such miniature cameras are constant, as are demands for continued miniaturization. In particular, high image quality requires the lens motion in a direction parallel to the optical axis to be accompanied by minimal parasitic motion in the other degrees of freedom. As a result, the lens motion is limited to single degree of freedom, for example in a direction parallel to the optical axis, with no tilt about axes orthogonal to the optical axis. This requires the lens suspension mechanism to be stiff to such parasitic motions. However, given the need to control the lens position to around 1 micron, such suspension mechanisms must also account for friction.
Various types of autofocus actuators have been proposed for use in miniature cameras. One exemplary autofocus actuator is a piezoelectric actuator, which uses ultrasonic vibrations to drive lens movement. Existing piezoelectric actuators, however, are relatively large in size and costly to manufacture.